1. Field of the Invention
The invention relates to a process for the separation and purification of an aqueous reaction mixture comprising the main components acetic acid and formic acid.
2. The Prior Art
The preparation of acetic acid by catalytic oxidation of saturated and/or unsaturated hydrocarbons, for example the gas-phase oxidation of C4-hydrocarbons, results in formation of product streams comprising as main components acetic acid, formic acid and water in varying proportions.
To work them up further, these product streams have to be separated into their individual components. Separation of a ternary acid/water mixture comprising acetic acid, formic acid and water into its pure components by distillation, for example, presents considerable problems since the system contains not only the binary water/formic acid maximum azeotrope but also a ternary water/formic acid/acetic acid saddle azeotrope.
If such a mixture has a high water concentration, separation by distillation has a tremendous additional energy requirement since all the water has to be distilled off at the top of a column as lowest-boiling component.
For the separation of aqueous mixtures having an acetic acid content of  greater than 60% by weight and a formic acid content of 5% by weight, Hunsmann and Simmrock (Chemie-Ing.-Tech., 38, 1966) recommend the use of azeotropic distillation for making the separation easier and for reducing the energy required. As azeotropic entrainer for the removal of water, ethyl n-butyl ether is proposed. The azeotrope of water and entrainer boils at about 91xc2x0 C. and contains about 10% by weight of water. The entrainer ethyl n-butyl ether forms no azeotrope with formic acid and acetic acid.
For separating off formic acid, DE-A 1204214 recommends azeotropic rectification using n-butyl chloride as entrainer. The disadvantage of this process is the use of chlorinated hydrocarbons as entrainer.
U.S. Pat. No. 5,633,402 discloses a process for the separation of binary mixtures of formic acid and acetic acid by means of azeotropic distillation. Methyl formate is used as entrainer for the formic acid. Removal of water is not described in this process.
DE-A 4426132, EP-A 0635474, DE-A 19610356 (U.S. Pat. No. 5,662,780) disclose various processes for the purification and dewatering of acetic acid by means of azeotropes with various entrainers. However, none of these processes describes the dewatering of a mixture of acetic acid and formic acid.
U.S. Pat. No. 5,173,156, U.S. Pat. No. 5,006,205, U.S. Pat. No. 4,877,490 and U.S. Pat. No. 4,935,100 disclose processes for the dewatering of formic acid by means of extractive rectification. Entrainers mentioned here are, for example, cyclohexanone, oxalic acid, decanoic acid and methyl salicylate.
EP-A 156309 (CA-A 1238919) and EP-A 12321 (U.S. Pat. No. 4,262,140) describe the dewatering of formic acid by extractive rectification using carboxamides as auxiliaries. However, none of these processes describes the dewatering of a mixture of acetic acid and formic acid.
The xe2x80x9cProcess Economics Programxe2x80x9d Report No. 37A (1973) of the Stanford Research Institute discloses a process for the separation of an aqueous mixture comprising about 42% by weight of acetic acid and 2% by weight of formic acid. In this process, the aqueous mixture is concentrated by countercurrent extraction with diisopropyl ether. In the dewatering and solvent recovery column, the water is distilled off at the top as an azeotrope of water and diisopropyl ether. The bottom product, namely a mixture of acetic acid and formic acid containing about 0.12% by weight of water, is fractionated further by azeotropic rectification. Benzene is used as entrainer for the formic acid. A great disadvantage of this process is the low quality of the formic acid separated off, which still contains about 1% by weight of acetic acid, about 2% by weight of water and about 7% by weight of benzene. The use of benzene in this process and the residual benzene content in the formic acid make this process unattractive.
All the processes known from the prior art are either only suitable for satisfactorily separating binary mixtures such as acetic acid/water, formic acid/water and acetic acid/formic acid or only economically applicable to aqueous acid mixtures in which a high concentration of acid ( greater than 60% by weight) is present. Furthermore, some of the known processes are no longer acceptable from the point of view of today""s safety and environmental standards because of their use of benzene or chlorinated hydrocarbons.
It is therefore an object of the invention to provide a process for the separation of a ternary, aqueous mixture of acids into its pure components, which process does not have the disadvantages mentioned in the discussion of the prior art.
It has now been found that the separation and purification of a mixture comprising the main components acetic acid, formic acid, water and high boilers (hereinafter referred to as crude acid) can be carried out particularly readily if the mixture is extracted by means of a solvent in a circulation process in a first step and the extract stream consisting predominantly of solvent, acetic acid, formic acid, high boilers and water is subsequently fractionated in a sequence of distillation steps into the constituents solvent which is recirculated to the extraction, water, formic acid, acetic acid and high boilers, and the raffinate stream is freed of solvent in a further distillation step by means of a solvent stripping column.
The invention provides a process for the separation and purification of an aqueous mixture comprising the main components acetic acid, formic acid and high boilers by extraction with a solvent in a circulation process, which comprises feeding the raffinate stream containing a major part of the water to a solvent stripping column (11) for removal of the water and conveying the extract stream to a solvent distillation column (8) from which, in a first step, a mixture (A) comprising the major part of the solvent (line (1)) is separated off via the top and a mixture (B) comprising formic acid, water and solvent is separated off via a side offtake and a mixture (C) comprising acetic acid and high boilers is separated off via the bottom, and, for further processing, conveying (line (2)) the mixture (B) to a formic acid distillation column (4) and conveying (line (3)) the mixture (C) to an acetic acid distillation column (5), subsequently isolating the pure acetic acid at the top of the acetic acid distillation column (5), isolating the pure formic acid at the bottom of the formic acid distillation column (4) and at the top taking off a mixture of solvent and water which, together with the mixture (A) after separating off the water present, is recirculated to the extractor (7).